1. Field of the Invention
The present invention relates to a method for decoding at least one signal transmitted by means of at least one transmitting antenna and received by means of at least one receiving antenna, which method includes a symbol decoding step for producing estimated symbols representative of at least one transmitted symbol carried by the received signal, and likelihood values associated to said estimated symbols, which estimated symbols are identified among predetermined symbols forming a lattice constellation of symbols which may potentially be received by means of said receiving antenna, said estimated symbols being included in a sphere having a predetermined radius.
2. Description of the Related Art
Such signals are exchanged, for example, in telecommunication systems of a Multiple Input Multiple Output type, further referred to as MIMO systems. A main feature of MIMO systems lies in the fact that a plurality of antennas may be used both at a transmitter end and at a receiver end of a wireless link. MIMO systems have been shown to offer large transmission capacities compared to those offered by single antenna systems. In particular, MIMO capacity increases linearly with the number of transmitting or receiving antennas, whichever the smallest, for a given Signal-to-Noise Ratio (SNR) and under favourable uncorrelated channel conditions. Specific coding schemes have been designed to exploit such an increased available transmission capacity. These schemes, called space-time codes, mainly aim at transmitting signals that are redundant in space and time, which means that a same information shall be transmitted over several antennas and several times, in order to benefit from the spatial diversity offered by the multiple antennas. Several types of space-time codes, designed according to various criteria, can be found in the literature.
Due to the advantages described above, MIMO techniques are likely to be used in future wireless systems intended to provide large spectral efficiencies or, alternatively, reduce the transmitting power required for obtaining a spectral efficiency equivalent to that which is obtained in current telecommunication systems. Such MIMO techniques will very likely be combined with multi-carrier modulation techniques like OFDM (standing for Orthogonal Frequency Duplex Multiplex) and MC-CDMA (standing for MultiCarrier-Code Division Multiple Access), which are also likely to be used in future wireless systems. In specific embodiments of MIMO systems, the information to be transmitted may be encoded with respect to space and time in a manner allowing to use only one antenna at the receiver end.
In the present state of the art, several aspects of space-time encoded MIMO systems are still open issues, such as symbol decoding schemes to be used on the receiver end of a signal transmitted by a transmitter using multiple antennas. Indeed, such a signal must be decoded by the receiver by means of a space-time decoder presenting a complexity which should be as low as possible, in order to spare computing power in a receiving device which is usually power-fed by a battery.
Among various existing decoding schemes, a so-called list sphere decoding technique may be singled out since it provides nearly optimal a posteriori probability decoding. The list sphere decoding scheme essentially consists in identifying, among predetermined symbols forming a lattice constellation of symbols which may potentially be received by means of at least one receiving antenna, estimated symbols which may represent the transmitted symbols. A metric representing the distance between the received symbol and a given estimated symbol of the lattice constellation constitutes the likelihood associated with said estimated symbol. In order to limit the extent of the search for such estimated symbols, only the most likely symbols of the lattice are examined, i.e. those closest to the received symbol, such a limitation being performed by only examining symbols which are included in a sphere having a predetermined radius and centred on the received symbol. Such list sphere decoding schemes have been described in European Patent applications EP 1 215 839 A1 and EP 1 221 773 A1.
A major problem encountered when implementing such a list sphere decoding technique lies in a proper choice of the initial radius of the sphere, which may have to be increased step by step until a suitable number of estimated symbols are identified. In particular situations which often occur in practice, the received symbol may be located outside the lattice constellation, so that the suitable radius of the sphere must have an important value, which will entail a high number of iterations in the course of which the sphere radius will be increased so that the sphere may encompass a suitable number of symbols of the lattice constellation. Such numerous iterations will require considerable computing power on the receiver end. Besides, a high final value for the sphere radius does not guarantee that the resulting sphere will include enough lattice constellation symbols for the symbol decoding step to produce a sufficiently high number of estimated symbols for said symbols to be statistically significant.